1. Field of the Invention
The invention is directed to a piston of a reciprocating expansion engine and to a reciprocating expansion engine for use in a steam circuit of a heat recovery circuit connected to an internal combustion engine in which a piston of this type can preferably be used. The piston has a piston head, a piston neck, and a piston shaft. The piston head has at least one groove which runs in a circumferential direction for receiving a piston ring. The piston shaft has a receptacle for a piston pin and, at its outer circumference, a guide surface for guiding the piston along an inner wall of a cylinder.
2. Description of the Related Art
In the development and advancement of internal combustion engines, work in recent years has focused mainly on minimizing pollutants and increasing the efficiency of the units. In this connection, one possibility for increasing efficiency in modern internal combustion engines is to use the heat generated in the area of the internal combustion engine. By taking appropriate measures, it is possible to limit the structural dimensions of the required cooling system and make use of the lost heat, which would otherwise merely be given off into the environment, for other applications within the motor vehicle.
To make the most efficient use of the heat lost in the area of an internal combustion engine, there is an increasing push toward the development of heat recovery systems within which the heat generated at the internal combustion engine is transferred to a heat transfer fluid and is rendered usable by a steam engine located in a secondary heat circuit.
Publication DE 34 29 727 A1 discloses a combustion/steam compound engine that uses process heat whereby fuel consumption is minimized, chiefly by a steam engine arranged downstream of the internal combustion engine. In the drive unit, which is referred to in this publication as a compound engine, a portion of the waste heat generated in the internal combustion engine is transferred to the coolant water which is brought to boiling. The steam generated by the boiling coolant water is then stored under pressure in a steam boiler and is made available subsequently to a steam engine. The driving torque generated by the steam engine is transmitted to a crankshaft of the internal combustion engine.
EP 1 870 575 A2 describes a charged internal combustion engine of a motor vehicle having a cooling circuit in which a work medium circulates and is at least partially changed to a vaporous or gaseous aggregate state. In this case, an expander unit is provided in operative connection with a driven shaft of the internal combustion engine by a drivetrain. In the expander unit described in this reference, a driven shaft of the expander unit is moved by a conversion of the energy contained in the at least partially vaporous or gaseous work medium. The expander unit described in this reference is constructed as a two-stroke piston engine in operative connection, indirectly or directly by means of the drivetrain, with the driven shaft of the internal combustion engine.